We have been investigating the transformation of Mg2SiO4
spinel (γ) to modified spinel (ß)
in the multianivil press in order to examine the mechanism of this phase
transition. These two phases can potentially undergo phase transitions
by a diffusionless, shear-like mechanism in which a reconfiguration the
stacking of the basic spinelloid units occurs. Phase transformations which
occur by this type of mechanism may result in a significant decrease in
shear strength as the transformation occurs, an effect described as transformation
plasticity. The operation of this mechanism during phase transformations
in subducting oceanic slabs, for example, has the potential to change the
rheological properties of the slab in the region where the transformation
occurs. The spinel starting material was synthesized by reacting synthetic
forsterite at 20.5 GPa and 1200 °C. After quenching to room temperature,
this material was recovered and run in a second set of experiments at various
conditions of temperature and pressure within the ß-phase
stability field. Pressure was increased at room temperature before the
sample was heated to the run temperature. Experiments have been carried
out at 900 °C, 15 GPa; 1000 °C, 15.5 GPa; 1100 °C, 16 GPa and
1200°C, 16.5 GPa for different periods of time. TEM studies of the
starting material show that during pressurization to 15-16 GPa significant
dislocation development has occurred in some grains, but in general the
samples are undeformed. There is no evidence of transformation to ß-phase
during this pressurization step. After 45 minutes at 900 °C, spinel
has developed a complex microstructure due to the development of a high
density of stacking faults on {110}, which result from a shear-like transformation.
Electron diffraction patterns of the spinel show no evidence of discrete
diffraction maxima which can be indexed as ß-phase,
so this phase appears to be an intermediate disordered spinelloid phase.
After 90 minutes at 900 °C, some spinel grains give diffraction patterns
which clearly show that they are intergrowths of both spinel and ß-phase,
in the crystallographic orientation relationship consistent with a shear
transformation. At temperatures 1000
°C, the transformation mechanism is significantly different. After
10 and 60 minutes at 1000 °C, there is no evidence of any disordering
of spinel. Instead, discrete ß-phase crystals
have nucleated at grain boundaries and triple junctions and in many cases
are crystallographically oriented with respect to the spinel. At least
4 distinct orientation relationships have been observed. Experiments carried
out at 1100° and 1200 °C show the same type of microstructures
except that the number of grains which are crystallographically oriented
with respect to the spinel appears to decrease with increasing temperature.
These results demonstrate that there is a major change in the mechanism
of the γ to ß phase
transformation as a function of increasing temperature. At lower temperatures
the mechanism is dominated by shear-like transformation, but changes with
increasing temperature to a diffusion-controlled nucleation and growth
mechanism.